1. Field of the Invention
The object of the invention is the end part of an air dryer, and a method in the end part of an air dryer, an air dryer and use of a centrifugal fan according to the preambles of the independent claims presented below.
2. Description of Prior Art
A typical air dryer according to prior art suitable for drying a pulp web, such as a cellulose web or the like, comprises                an actual dryer section in which the pulp web is dried in air flotation drying with hot air jets or the like,        at least one, usually a large number of axial circulation air fans arranged adjacent to the actual dryer section with which circulation air fans humid circulation air is discharged from the actual dryer section and with which heated circulation air is blown as drying air into the actual dryer section, and        heating devices with which the discharged humid circulation air is heated before returning it as drying air into the actual dryer section.        
Several axial fans operating side by side are usually arranged on top of each other in order to create so called fan towers, a necessary amount of which fan towers, in turn, are arranged side by side. The fans are mounted in the fan tower so that their suction openings are confined to the front and back side of the fan tower, seen from the travelling direction of the web to be dried. Heating devices are arranged on the front and back sides of the fan towers, in front of the suction openings of the fans, which heating devices heat the humid air discharged from the dryer section before the fans. A single fan of the fan tower is arranged to blow drying air to a dryer group, which consists of a number of drying means, such as nozzle boxes, extending over the web to be dried. The longitudinal direction of these drying means is thereby essentially perpendicular against the travelling direction of the web to be dried. Respectively, the drying level of the air dryer comprises a number of such adjacently arranged dryer groups.
The so-called roll towers are located in the ends of the air dryer. A number of turn rolls are arranged in the roll towers mainly on top of each other, with which turn rolls the web to be dried is turned from one drying level to the next one. At the same time the travelling direction of the web changes by 180°. The axis of the rolls are arranged parallel in the roll towers.
The area between the roll tower and the fan tower located nearest to it has proven to be problematic. Typically, due to physical size limitations, an axial fan of the size used in other fan towers of the air dryer can not be placed in said area. Due to lack of space, air intake of the fans in the fan tower located nearest to the roll tower has been problematic, sometimes even insufficient.
In the area between the roll tower and the fan tower located nearest to it, after the last dryer group, expensive auxiliary level nozzles of special structure have been arranged in order to convey the pulp web to the turn roll of the roll tower. These auxiliary level nozzles have been fed with air from the axial fans of said nearest fan tower. These axial fans have thus been used to arrange air both for the drying means of their “own” dryer groups, as well as for the auxiliary level nozzles located as an extension of the drying levels. As a consequence, the blowing capacity of the axial fans has not necessarily been sufficient to entirely satisfy the created need for blowing. Thereby the drying capacity of the dryer groups located nearest to the roll tower has decreased compared to other dryer groups of the air dryer. The amount of air fed to the auxiliary level nozzles has again not been sufficient to maintain the same blowing velocity between the roll tower and the nearest, adjacent fan tower as in the other parts of the air dryer. This has led to a generally poor drying capacity in the end parts of the air dryer. Problems have also appeared in the process if the air jets from the auxiliary level nozzles have not been able to support the pulp web to be dried, but have allowed it to “drag”. This has also caused dusting problems of the web.
The end parts of the air dryers nowadays in use may have parts of even two metres long wherein the drying capacity is low due to the use of an auxiliary level nozzle. The low drying capacity leads indirectly also to an increase of the total size of the air dryers. Large air dryers occupy expensive mill room and increase the expenses of the mill investments.